1. Field of the Invention
The present invention relates to an air-fuel ratio control system for an internal combustion engine.
2. Description of the Prior Art
The applicant of the present application has proposed an air-fuel ratio control system for controlling the air-fuel ratio of an internal combustion engine, more accurately, the air-fuel ratio of an air-fuel mixture to be combusted by the internal combustion engine, in order to enable a three-way catalytic converter disposed in the exhaust system of the internal combustion engine. See, for example, Japanese laid-open patent publication No. 9-273439 (U.S. Pat. No. 5,845,491).
According to the disclosed technology, an exhaust gas sensor (O.sub.2 sensor) for detecting the concentration of a certain component of an exhaust gas that has passed through a catalytic converter, e.g., the concentration of oxygen, is disposed downstream of the catalytic converter. The air-fuel ratio of the internal combustion engine is manipulated to equalize an output from the exhaust gas sensor (a detected value of the concentration of oxygen) to a predetermined target value (constant value) for thereby enabling the catalytic converter to achieve an optimum purifying capability irrespective of aging of the catalytic converter.
More specifically, a manipulated variable for manipulating the air-fuel ratio of the internal combustion engine is successively generated in given control cycles in order to converge the output of the exhaust gas sensor to its target value based on a feedback control process (specifically, a sliding mode control process) with a manipulated variable generating means implemented by a microcomputer. Based on the manipulated variable, the amount of fuel supplied to the internal combustion engine is regulated to manipulate the air-fuel ratio of the internal combustion engine. In this manner, the internal combustion engine is controlled to emit an exhaust gas for converging the output of the exhaust gas sensor to its target value, and achieving an optimum purifying capability of the catalytic converter.
According to the above publication, the manipulated variable generated by the manipulated variable generating means represents a target value for the difference between a target air-fuel ratio for the internal combustion engine or an air-fuel ratio of the internal combustion engine and a predetermined reference value (which difference is the difference between the target air-fuel ratio and the reference value). However, the manipulated variable may alternatively be a corrective variable for the amount of fuel supplied to the internal combustion engine, or a quantity that can define the air-fuel ratio of the internal combustion engine.
In the above publication, the exhaust gas sensor disposed downstream of the catalytic converter comprises an O.sub.2 sensor. However, the exhaust gas sensor may comprise an NOx sensor, a CO sensor, an HC sensor, or another exhaust gas sensor. It is possible to enable the catalytic converter to have a desired purifying ability by controlling the air-fuel ratio of the internal combustion engine so as to converge the output of such an exhaust gas sensor to a suitable target value.
The manipulated variable generated by the manipulated variable generating means according to the feedback control process to converge the output from the exhaust gas sensor disposed downstream of the catalytic converter to the target value for enabling the catalytic converter to have a desired purifying ability suffers a large variation due to disturbance, e.g., suffers a spike-like variation. If the air-fuel ratio of the internal combustion engine is manipulated directly using the manipulated variable generated by the manipulated variable generating means, then the air-fuel ratio may be excessively be varied, tending to make unstable the operating state of the internal combustion engine.
In view of the above drawback, it has been customary to set an allowable range for the manipulated variable used to actually manipulate the air-fuel ratio of the internal combustion engine to a fixed range. After the generated manipulated variable is limited to a value in the allowable range, the air-fuel ratio of the internal combustion engine is manipulated based on the limited manipulated variable. Specifically, when the generated manipulated variable exceeds the upper limit value or lower limit value of the allowable range, the manipulated variable is forcibly limited to the upper limit value or lower limit value of the allowable range.
The limiting process is effective to avoid excessive changes of the air-fuel ratio of the internal combustion engine depending on the manipulated variable, allowing the internal combustion engine to operate stably.
However, further studies made by the inventors of the present application have revealed that the fixed allowable range for the manipulated variable results in the following shortcomings:
A range (variation range) of values of the manipulated variable generated by the manipulated variable generating means for determining an air-fuel ratio required to converge the output of the exhaust gas sensor disposed downstream of the catalytic converter to a predetermined target value, or a central value in the range is variable by various factors including operating conditions (e.g., the atmospheric pressure, intake air temperature and humidity, rotational speed, etc.) of the internal combustion engine, the nature of the fuel of the internal combustion engine, states (temperature, deterioration, etc.) of the catalytic converter.
Therefore, even when the range of values of the manipulated variable generated by the manipulated variable generating means is stable and less subject to the effect of disturbances, the range may be displaced toward the upper or lower limit of the fixed allowable range, i.e., the central value of the range of values of the generated manipulated variable may be offset from the central value of the allowable range.
In such a case, even though the manipulated variable generated by the manipulated variable generating means poses no problem in stably operating the internal combustion engine, the manipulated variable limited by the above limiting process is highly frequently limited to the upper or lower limit of the allowable range. Occasionally, the manipulated variable may be continuously held to the upper or lower limit of the allowable range.
The manipulated variable forcibly limited by the limiting process differs from the manipulated variable generated by the manipulated variable generating means in order to converge the output from the exhaust gas sensor to the target value, i.e., the manipulated variable defining the air-fuel ratio of the internal combustion engine required to converge the output from the exhaust gas sensor to the target value. Consequently, when the manipulated variable limited by the above limiting process is highly frequently limited to the upper or lower limit of the allowable range, or the manipulated variable is continuously held to the upper or lower limit of the allowable range, the controllability of the output from the exhaust gas sensor at the target value is impaired, i.e., the quick response and stability of the control process for converging the output from the exhaust gas sensor to the target value are lowered, making it difficult to achieve a desired purifying ability of the catalytic converter.
If the range of values of the manipulated variable generated by the manipulated variable generating means is displaced toward the upper or lower limit of the fixed allowable range, then when the manipulated variable contains a spike and is temporarily largely changed to one of the upper and lower limits due to disturbances, the spike manipulated variable often cannot be properly limited by the limiting process. As a consequence, the air-fuel ratio defined by the manipulated variable suffers an excessive change, impairing the stability of operation of the internal combustion engine.